Method for producing an opening in a metallic structural part

ABSTRACT

The invention relates to a method of producing an aperture in a metallic component, in which the aperture comprises, at least in certain portions, a non-cylindrically formed funnel, extends from a first surface to a second surface of a component wall and is formed by a laser beam. The invention improves the dimensional accuracy and reduces the roughness of the surface of the aperture and/or funnel in that, by appropriate choice of the laser parameters, the metal is predominantly removed by sublimation during the formation of the funnel and the funnel is formed by laser removal of layers of a substantially constant thickness.

This application claims the priority of German Patent Document No. 19960 797.4, filed Dec. 16, 1999, and PCT International Application No.PCT/DE00/04422, filed Dec. 13, 2000, the disclosures of which areexpressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method of producing an aperture in a metalliccomponent, in which the aperture comprises, at least in certainportions, a non-cylindrically formed funnel, extends from a firstsurface to a second surface of the component and is formed by a laserbeam.

EP-A-0950463 discloses a method of forming a cooling hole comprising adiffuser portion, in which the diffuser portion is cut out by abeam-drilling method in such a way that the drilling beam remains withina previously selected cross-sectional area.

It is known from S. NOLTE, G. KAMLAGE: ‘Mikrostrukturierung mitFemtosekundenlasern’ [Microstructuring with femtosecond lasers],LASEROPTO, Vol. 31, No. 3 Apr. 15, 1999 (1999-04-15), pages 72-76,XP000999012, that, during laser machining, an increasingly shorter pulseduration and a reduction in the ambient pressure assist the sublimationof the material and lead to an improved workpiece surface. In particularduring laser-beam drilling, the striation of the hole and the drillingcore can be improved by a reduction in the ambient pressure and byspecial process gases, such as helium.

U.S. Pat. No. 5,609,779 discloses a method of laser drillingnon-circular apertures in a metallic component, in which the aperturecomprises a diffuser which extends up to one surface of the componentand is formed by vaporizing the metal by means of a laser, the laserbeam traversing the surface from a center line of the diffusertransversely to both sides at a respectively increasing rate and withoverlapping laser spots, to allow non-circular apertures to be producedwith a conventional laser as inexpensively as possible and with arelatively good surface. The increasing rate and the overlapping areintended to compensate for tolerances in the pulse energy which lead tovarying material removal, and at the same time produce the specific,cross-sectionally non-circular form of the diffuser.

In this operation, it has proven to be problematical that the metalbecomes liquid due to the pulse frequency and pulse duration used, whichhas adverse effects on the surface of the diffuser. Owing to the lasermachining at an increasing rate, the thickness of the layers varies anddecreases from the center line outwards, whereby cumulative inaccuracieswith regard to the form of the diffuser may occur, in particular in thecase of a number of successive layers.

The object of the present invention is to provide a method of thegeneric type described at the beginning with which apertures can beformed with the best possible surface and dimensional accuracy.

The solution to the problem is characterized according to the inventionin that, by choice of the laser parameters, such as for example thepulse frequency, pulse energy or pulse duration, the metallic materialis predominantly removed by sublimation, at least during the formationof the funnel, and the funnel is formed by laser removal of layers of asubstantially constant thickness.

It is advantageous in the case of material removal by sublimation, as aresult of the high energy input per pulse, that undefined deposits ofviscous material in the region of the aperture or the funnel to beremoved, which lead to increased roughness, are avoided. The definedremoval in layers of substantially constant thickness ensures highdimensional accuracy with low roughness of the surface of the apertureand/or of the funnel. The method can be carried out efficiently fromtechnical production-related aspects. Final finishing of the surface ofthe aperture or the funnel formed in this way is not required, savingtime and costs in the production process.

During the removal of the layer, the laser beam can be moved at aconstant rate in relation to the respective surface, beginning with anouter surface, of the component, in order in this way to achieve adefined removal of layers of substantially constant thickness. Therelative movement between the laser beam and the component to bemachined is generally produced by moving the component, which is clampedin a device of a suitable machine tool. Similarly, this can be achievedby a generally more restricted movement of the laser or a superimposedmovement.

Suitable laser parameters allow layers of substantially constantthickness of from 1 μm to 10 μm to be removed. The form and dimensionsof the successively removed layers can be adapted to the form of thefunnel, whereby the apertures can be produced economically without anyfinishing. The form of the funnel is substantially described by a firstaperture angle, determining a height H of the funnel, and a secondaperture angle, determining a width B. The funnel may alternatively alsobe conically formed with a circular cross section.

For removing the layers, a laser beam can be moved in a number ofneighboring, generally parallel, paths over the respective surface ofthe component, the removal beginning at an outer surface of the metalliccomponent and taking place, for example, line by line for each layer. Anoverlapping of the individual paths is not required and is avoided tocarry out the method as efficiently as possible.

To realize layers of substantially constant thickness, the laser beamcan move along the individual paths at a constant rate.

The laser removal can be carried out at a pulse frequency of from 1 to50 kHz, in order to remove the material predominantly by sublimationduring the forming of the aperture and/or the funnel. For this purpose,as much energy per pulse as possible must be introduced into the regionto be removed over an extremely short period of time. To realize theextremely short period of time, the laser removal can be carried outwith a pulse duration of from 10 to 1000 ns and a pulse energy of 0.005to 1 Joule per pulse, these laser parameters allowing pulse peak outputsin the range from 50 kW to 1 MW to be achieved.

Before or after the forming of the funnel, a cross-sectionallysubstantially circular or cylindrical (aperture) portion of the aperturecan be formed by laser drilling, so that the aperture comprises anon-cylindrically formed funnel and a substantially cylindrically formed(aperture) portion. During the laser drilling of the cylindrical(aperture) portion, the laser beam is aligned in the direction of thecenter axis of the aperture, generally running at an acute angle inrelation to the surface of the component.

During the removal of the layer, the laser beam can move spirally overthe respective surface of the component, it being possible for therelative movement to begin at the center, where the center axis of theaperture intersects the outer surface of the component, or at the outercircumference of the funnel.

Further exemplary embodiments of the invention are described.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below on the basis ofexemplary embodiments with reference to a drawing, in which:

FIG. 1 shows in a perspective representation a turbine blade of a gasturbine with apertures formed as cooling-air holes, which are producedaccording to a refinement of the method according to the invention,

FIG. 2 shows a cross-sectional view through a component wall in which acylindrical aperture portion produced in a first step according to arefinement of the method according to the invention is represented,

FIG. 3 shows a cross-sectional view through a component wall in which anaperture produced according to a refinement of the method according tothe invention is represented and

FIG. 4 shows a cross-sectional view through a component wall in whichanother aperture produced according to an alternative refinement of themethod according to the invention is represented.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows as a metallic component a turbine blade 1 of a gas turbine,such as for example an aircraft engine, in a perspective representation,in which numerous apertures 2, formed as cooling-air holes, are formedby the method according to the invention. The cooling-air holes 2generally run at an acute angle 4, shown in FIG. 2, through thecomponent wall 3, which angle usually lies in the range from 12° to 350°in relation to an outer surface 6 of the component 1 and is, forexample, 30°. From a cavity in the turbine blade 1, compressor air ispassed through the cooling-air holes 2, in order to direct a film ofcooling air over the outer surface 5 of the turbine blade 1.

The turbine blade 1 consists of a metal, such as for example an Ni- orCo-based alloy, and, for producing the apertures 2, is clamped in asuitable machine tool, in which it can be moved or turned along a numberof axes. The relative movement between a laser with which the forming ofthe aperture 2 is performed by laser removal and the component 1 to bemachined is generally produced by moving the component 1. Similarly,this can be achieved by a generally more restricted movement of thelaser or a superimposed movement.

FIG. 2 shows a sectioned view of the component wall 3 of the turbineblade 1, in which an aperture portion 7 produced in a first stepaccording to an exemplary embodiment of the method according to theinvention, having a substantially circular cross section, isrepresented. The center axis 8 of this cylindrical aperture portion 7runs at an acute angle 4 of approximately 30° through the component wall3. As a consequence of this, in the plan view indicated merely by dashedlines, the circular cross section is represented in an elongated form inthe inner, first surface 5 and the outer, second surface 6.

The apertures 2 produced by the method according to the invention arethrough-apertures and extend from the inner, first surface 5 to theouter, second surface 6 of the component wall 3. Before the forming of across-sectionally non-circular funnel 9, shown in FIG. 3, firstly, in afirst step, the aperture portion 7 of circular cross section is formedby means of laser drilling. For this purpose, a schematically indicatedlaser beam 10 of an Nd-YAG laser is used and directed onto the outer,second surface 6 of the component wall 3 in the direction of the centeraxis 8 of the aperture portion 7 to be drilled. The laser parameters,such as the pulse duration and pulse energy, as well as the number ofpulses, are chosen according to the thickness of the component wall 3.

In the case of an exemplary embodiment of the method according to theinvention, then, in a second step, the non-cylindrical funnel 9,represented in FIG. 3, is formed, respectively comprised by thecooling-air holes 2 for reasons of flow dynamics, in order to increasethe pressure of the air flowing through. For this purpose, by suitablechoice of the laser parameters, the metallic material is sublimatedduring the laser removal, beginning from the outer, second surface 6, sothat the metal substantially does not go into the liquid phase and noviscous material can be deposited in the region to be removed, therebyimpairing the quality of the surface.

The forming of the funnel 9 takes place by substantially parallel layers11, which in FIG. 3 are indicated schematically by dashed lines andgreatly enlarged, being removed with respect to the outer, secondsurface 6. The laser removal takes place with a multiplicity of pulsesof the laser beam 10, the pulse energy being adapted to the thickness ofthe layers 11 to be removed. Depending on the desired form of the funnel9, the second aperture angle 14 of which, determining a width B, canvary in relation to the center axis 8 and is, for example, approximately15°, the thickness of the layers 11 lies between 1 μm and 10 μm Thematerial of the individual layers 11 is respectively removed by laser inpaths 12 indicated by dashed lines and greatly enlarged, in which thelaser beam 10 moves line by line in relation to the surface 6 of thecomponent 1. The movement along the paths 12 takes place at a constantrate. The paths 12 respectively extend over the entire width B or, inthe case of another alignment, the entire height H of the funnel 9.

To ensure the sublimation of the metallic material of the component 1, ahigh pulse energy must be introduced by the laser beam 10 into theregion to be removed with an extremely short pulse duration. For thispurpose, the Nd-YAG laser is equipped with a Q-switch, and a pulseenergy of from 5 to 100 mJoules per pulse, with a pulse duration of from10 to 1000 ns, is chosen. Alternatively, along with the Nd-YAG drillinglaser, a separate Q-switch Nd-YAG laser can be used.

In the plan view indicated by dashed lines in FIG. 3 of the outer,second surface 6 of the component wall 3, it can be seen that thedimensions of the funnel 9 are chosen such that a height H resultingfrom a first aperture angle 13 is smaller than the width B of the funnel9. In the case of the present exemplary embodiment, the height Hcorresponds to the diameter D of the aperture portion 7. The funnel 9has the same center axis 8 as the aperture portion 7 and consequentlyruns coaxially in relation to the latter. The funnel 9 and thecylindrical aperture portion 7 together form the aperture 2.

FIG. 4 shows in a cross-sectional view another aperture 2, formed as acooling-air hole, which is produced by an alternative refinement of themethod according to the invention. It is also the case with thiscooling-air hole 2 that, firstly, an aperture portion 7 of substantiallycircular cross section is formed by laser drilling at an acute angle 4of approximately 30° in relation to the outer, second surface 6 of thecomponent wall 3 of the turbine blade 1. The laser beam 10 of the Nd-YAGlaser is in this case aligned in the direction of the center axis 8 ofthe aperture portion 7.

In a second step, the funnel 9 is then formed by laser removal,beginning from the outer, second surface 6 of the component wall 3. Inthis case, the Nd-YAG laser is equipped with a Q-switch and, byappropriate choice of the laser parameters, is capable of sublimatingvirtually completely the metallic material to be removed for the formingof the funnel 9. Alternatively, for the second step, a separate Q-switchNd-YAG laser can be used along with the Nd-YAG drilling laser used inthe first step. In order to introduce into the material to be removedthe pulse peak output required for this purpose, in the range from 50 kWto 1 MW, a pulse energy of from 0.005 to 0.1 Joule per pulse and a pulseduration of 10 to 1000 ns are provided.

The thickness of the respective layer 11 is chosen according to the formof the funnel 9. This thickness generally lies in the range from 1 μm to10 μm. In the plan view indicated by dashed lines in FIG. 4, it can beseen that a width B of the funnel 9 extends substantially symmetricallyin relation to the center axis 8 of the aperture 2. The height H of thefunnel 9 extends asymmetrically in relation to the center axis 8, since,as can be seen well in the cross-sectional view of FIG. 4, the firstaperture angle 13 in relation to the center axis 8 towards one side islarge and is approximately 16°. As a result, the funnel 9 deviates awayfrom the center axis 8 of the aperture 2 more strongly on one side.

The layers 11 are respectively removed in the paths 12 indicated bydashed lines, in which the laser beam 10 moves at a constant rate inrelation to the component 1. The thickness of the layers 11 is adaptedin each exemplary embodiment of the method according to the invention tothe form of the funnel 9, which is substantially described by the firstaperture angle 13, determining the height H, and the second apertureangle 14, determining the width B. The funnel 9 may also be formedconically with a circular cross section.

In all the refinements of the method according to the invention, thefunnel 9 may be initially produced at the outer, second surface 6 of thecomponent wall 3 by laser removal by means of an Nd-YAG laser equippedwith a Q-switch and, following that, in a second step, the apertureportion 7 of substantially circular cross section may be produced bylaser drilling with an Nd-YAG laser, the funnel 9 and the cylindricalaperture portion 7 here again having a common centre axis 8. The form ofthe funnel 9 may be symmetrical or asymmetrical in relation to thecenter axis 8, depending on the requirement, as a result of the choiceof the aperture angle 13 and its dimensions H and B.

In a further refinement of the method according to the invention, thematerial of the individual layers 11 may be removed by the laser beam 10moving spirally around the center axis 8 of the aperture 2 in relationto the outer surface 6 of the metallic component 1.

What is claimed is:
 1. A method of producing an aperture in a metalliccomponent, in which the aperture comprises, at least in certainportions, a non-cylindrically formed funnel, extends from a firstsurface to a second surface of a metal component wall and is formed by alaser beam, wherein, by choice of laser parameters, the metal ispredominantly removed by sublimation during the formation of the funneland the funnel is formed by laser removal of layers of a substantiallyconstant thickness, and wherein, during the removal of the layers, alaser beam moves in a number of neighboring paths, adapted in length andnumber to a form of the funnel, over a respective surface of thecomponent.
 2. The method according to claim 1, wherein, during theremoval of the layers, the laser beam moves at a constant rate inrelation to the respective surface of the component wall.
 3. The methodaccording to claim 1, wherein the layers are removed with asubstantially constant thickness of from 1 μm to 10 μm.
 4. The methodaccording to claim 1 wherein a form and dimensions of the layers areadapted to the form of the funnel.
 5. The method according to claim 1,wherein the laser beam moves along the neighboring paths at a constantrate.
 6. The method according to claim 1, wherein the laser removal iscarried out at a pulse frequency of from 1 to 50 kHz.
 7. The methodaccording to claim 1, wherein the laser removal is carried out with apulse duration of from 10 ns to 1000 ns.
 8. The method according toclaim 1, wherein the laser removal is carried out with pulse peakoutputs in the range from 50 kW to 1 MW.
 9. The method according toclaim 1, wherein before or after the forming of the funnel, across-sectionally circular portion of the aperture is formed by laserdrilling.
 10. The method according to claim 1, wherein during removal ofthe layers, the laser beam is moved in a spiral path.
 11. A method ofproducing an aperture including a non-cylindrical funnel in a metalliccomponent, comprising the steps of: forming the funnel by removing metallayers of a substantially constant thickness from a surface of thecomponent by sublimation by moving a laser beam along a number ofadjacent paths, the paths defined in length and quantity in relation toa desired form of the funnel.
 12. The method of claim 11, wherein duringremoval of the metal layers, the laser beam moves at a constant rate inrelation to the surface of the component.
 13. The method of claim 11,wherein the metal layers are removed with a substantially constantthickness of from 1 μm to 10 μm.
 14. The method of claim 11, wherein aform and dimension of the metal layers are adapted to the desired formof the funnel.
 15. The method according to claim 11, wherein the laserbeam moves along the neighboring paths at a constant rate.
 16. Themethod according to claim 11, wherein the laser removal is carried outat a pulse frequency of from 1 to 50 kHz.
 17. The method according toclaim 11, wherein the laser removal is carried out with a pulse durationof from 10 ns to 1000 ns.
 18. The method according to claim 11, whereinthe laser removal is carried out with pulse peak outputs in the rangefrom 50 kW to 1 MW.
 19. The method according to claim 11, wherein beforeor after the forming of the funnel, a cross-sectionally circular portionof the aperture is formed by laser drilling.
 20. The method according toclaim 11, wherein during removal of the layers, the laser beam is movedin a spiral path.